Current mirror circuits are in common use and attempt to equalize current levels between a plurality of parallel loads. FIG. 1 (Prior Art) is an example of a conventional current mirror circuit. A circuit 10 comprises a current source 12, a primary branch 14 and a secondary branch 16. The primary branch 14 includes a load consisting of ten (10) light emitting diodes (LED), LED1 to LED10, connected to a collector and to a base of a bipolar transistor Q1. The secondary branch 16 includes a load consisting of ten (10) other LEDs, LE11 to LED20, connected to a collector of another bipolar transistor Q2. A base of the transistor Q2 is connected to the collector of the transistor Q1. Emitters of the transistors Q1 and Q2 are connected to a common ground (GND). A current I0 flowing from the current source 12 is split into currents I1 and I2 that respectively flow in the primary branch 14 and in the secondary branch 16. The currents I1 and I2 also respectively flow through the transistors Q1 and Q2 and return to a ground (GND) connection of the current source 12.
The connection of the base of the transistor Q2 to the collector of the transistor Q1 is intended to equalize magnitudes of the currents I1 and I2. The skilled reader will readily appreciate that the circuit 10 may fail to ensure that the currents I1 and I2 are perfectly equal, as may be the case when the base to emitter voltage of the transistor Q1 is not equal to the base to emitter voltage of the transistor Q2.
FIG. 2 (Prior Art) is another example of a conventional current mirror circuit. A circuit 20 includes the same current source 12, the primary branch 14 and the secondary branch 16. The circuit 20 also includes further secondary branches 22, 24, 26 and 28 that respectively include loads formed of LED21 to LED30, LED31 to LED40, LED41 to LED50 and LED51 to LED60. The secondary branches 22, 24, 26 and 28 also respectively include transistors Q3, Q4, Q5 and Q6. The current I0 from the current source I2 is split into currents I1, I2, I3, I4, I5 and I6 flowing through the branches 14, 16, 22, 24, 26 and 28. Bases of the transistors Q3, Q4, Q5 and Q6 are also connected to the collector of the transistor Q1, so that the currents I1-I6 are intended to have substantially equal magnitudes.
FIG. 3 (Prior Art) is a further example of a conventional current mirror circuit. A circuit 30 differs from the circuit 20 in that, in branches 34, 36, 38, 40, 42 and 44, feedback resistors R1, R2, R3, R4, R5 and R6 are respectively connected between emitters of the transistors Q1, Q2, Q3, Q4, Q5 and Q6 and the common ground (GND). The presence of the feedback resistors R1 to R6 tends to mitigate variations of base to emitter voltages between the transistors Q1 to Q6. Generally, the feedback resistors R1 to R6 have substantially the same value.
In FIGS. 1, 2 and 3, the loads as illustrated consist of LEDs. Current mirrors may also be used to equalize currents between many other types of loads.
A problem with conventional current mirrors as illustrated in FIGS. 1, 2 and 3 is that current equalization is far from being perfect, particularly when current varies over time. Current imbalances may persist when using any one of the circuits 10, 20 or 30.
Therefore, there is a need for improvements that compensate for problems related to potential current imbalances in current mirror circuits.